This invention relates to fluorescent lamp circuits of the preheat type which employ starting filaments and in which lamp operating current is controlled by an inductor ballast. Such lamp circuits are commonly used, and are achieving increased use as replacements for the less energy efficient incandescent light bulbs in residential lighting fixtures, portable lamps and fluorescent lampholder fittings, such as those exemplified by my U.S. Pat. No. 4,161,020.
In such residential applications it is very desireable that lamps start quickly and positively. However, currently used preheat inductor ballasts do not start quickly, but instead go through a series of flickering attempts to start before normal lamp operation is achieved. In a typical starting cycle the starting switch may operate from five to ten cycles before current flow through the lamp is established and the lamp is in a normal running mode. Further, where energy consumption is important, it is common practice to produce "energy saving ballasts" which operated the lamp at less than rated current, further degrading the starting characteristics of the lamp. The operating lifetime of fluorescent lamps is based to a great extent on the number of starts involved. Although the lamp manufacturers rate lamps in terms of hours of operation, the standard includes the number of hours per start. In typically used residential lamps, such as the 22-watt circline lamp rated for 12,000 hours at 31/2 hours per start, the effective lamp life is actually 3,428 starts or approximately 10 years of daily use. However, if the ballast design permits the starter to attempt to start the lamp five times before normal running is achieved, the real lamp life is only 2 years.
Similarly, the lamp starter is rated for 6,000 starts to be rated as certifiable quality. This would represent 16 years of daily use, but it is well known that the starter is the most likely component to fail in a preheat lamp circuit. In this case also, each attempted start must be included to obtain the real expected starter life, which is usually no more than three years.
The function of the fluorescent ballast is simply to limit the current flow through the lamp to prevent the lamp current from the thermal runaway in which the lamp would destroy itself. A specific lamp may be operated over a wide range of lumen output by varying the current limiting of the ballast. Therefore ballasts have become available which may vary .+-.20% from the rated lamp current, and which also have respective proportional starting currents. Fluorescent ballast designers have in the past been very careful to avoid inductor core saturation for both the starting and running modes of preheat type inductor ballasts. Therefore the higher running current ballasts have higher starting current, and tend to start more quickly. Conversely, the lower running current ballasts which are attractive as energy conservation devices, also have lower starting current with the attendant slow, flickering starts.
One application where reduced lamp brightness is needed is in multi-level lighting circuits, such as the three-way brightness circuit shown in my U.S. Pat. No. 4,178,535. It is mandatory in this type of application that the start be immediate, even if the circuit is in the low brightness mode, particularly since the first position of a three-way switch is always the low position. The use of presently known ballasts is not possible where the low brightness mode is 50% to 60% of full rated lamp current and lumen output, because the ballast will not provide sufficient starting current.
The inability of preheat inductor ballasts to start fluorescent lamps at low temperatures is well known in the lighting industry. Specifications for such ballasts limit operation to temperatures above 40.degree. Fahrenheit, with some manufacturers limited to above 50.degree. Fahrenheit. This ballast limitation restricts preheat lamps to indoor use even in moderate climates, and even precludes many indoor uses, such as garages, warehouses, etc.